Magnetron tube shield



Aug. 2, 1960 T. A. HADDAD MAGNETRON TUBE SHIELD Fiied March 23, 1956 United States Patent O MAGNETRON TUBE SHIELD Theodore A. Haddad, Swarthmore, Pa., assignor to Burroughs Corporation, Detroit, Mich., a corporation of Michigan Filed Mar. 23, 1956, Ser. No. 573,390

4 Claims. (Cl. 313-157) The invention hereinafter described and claimed relates to magnetron tubes. More particularly it has to do with shield means for magnetron beam switching tubes enabling their stable operation when in close proximity to other magnetic apparatus.

Multiple position beam switching tubes are a comparatively recent development in the tube art and industry is fast finding them useful in an increasing number and diversity of applications. However, due to the strong magnetic eld in which the tube works it has been found that surrounding apparatus may adversely be affected if placed too close to the tube. It is common practice where magnetic apparatus is used, to place a low reluctance shield around the apparatus to intercept the magnetic linx thus to prevent it from spreading over too wide an area. Usually, these devices are in the shape of an open ended can of high permeability metal which is slipped over the apparatus.

With magnetron beam switching tubes, however, in which it is important that a high ilux be maintained through the tube along lines parallel to the tubes axis, use of a shield of this general type would provide a convenient path for a large portion of the flux to bypass through the can walls and, thus, rob the tube of a large portion of this important axially aligned ilux. Therefore, it can be seen that use of the can only, would prove extremely detrimental to the stable operation of the tube.

Other factors to be considered are that these tubes are somewhat light sensitive and undesirable photoelectric effects are produced in them by incident light from surrounding area, and as is the case with many glass envelope tubes, the exhaust stem tip-off is exposed to accidental breaking, an undesirable condition in such complex and expensive tubes.

'Ihe present invention has for its primary object to provide means overcoming the above mentioned di'iculties.

It is also an object of the invention to provide means assuring stable operation of magnetron tubes placed in close proximity to other magnetic structure.

Another object of the invention is to provide shield means for beam switching tubes to prevent undesirable photo-electric effects in the tube, and simultaneously to protect the exhaust stem tip-off.

A more specific object is to provide such means which is of inexpensive construction and which is easily applied to the tube without altering its normal conguration and construction.

While the invention is 'of broader applicability it is particularly useful with magnetron beam switching tubes of the type 'shown and described in United States Patent 2,721,955 to Sin-pih Fan et al. While the tube is described in more detail hereinafter, exteriorly at least, it comprises a glass envelope mounted on a plastic base provided with the usual connector pins. Surrounding the tube and secured thereto by a suitable potting compound is a cylindrical permanent magnet providing a high axially aligned ilux through the tube.

In accordance with the invention and iirst brieily described, the shield may comprise low reluctance metallic shields positioned at the tube ends and having peripheral edge portions positioned adjacent to the ends of the cylindrical tube magnet.

The invention will be clearly understood from the following description with reference to the accompanying drawings wherein;

Figure 1 is an exploded perspective View of a magnetron beam switching tube including shielding elements in accordance with the present invention;

Figure 2 is an elevational view of the beam switching tube showing the shields and the tube magnet in section;

Figure 3 is a diagrammatic view of a beam switching tube including a schematic representation of the normal magnetic flux lines;

Figure 4 is a diagrammatic view similar to Figure 3 but schematically showing how the magnetic flux lines are affected when a low reluctance shield is placed over the tube;

Figure 5 is another diagrammatic view showing the magnetic ilux lines when the tube is shielded in accordance with the concepts of the present invention; and

Figure 6 is a fragmentary view of the end of a magnetron tube illustrating a modied shield construction.

Since the principles of the invention are particularly applicable to magnetron tubes of the multi-position beam switching type, the following detailed description and the accompanying drawings are illustrative of such use.

Now making more detailed reference to the drawings and initially to Figure l thereof, it is to be understood that this figure depicts a multi-position beam switching tube of the type shown in the above identified patent, and identified in this figure by the numeral 20. As mentioned above, this type of tube introduces special problems when used in close association with adjacent equipment, and in this regard the following brief description of the tube should be helpful in reaching a complete understanding of the invention.

The use of crossed electric and magnetic fields, and a number of electrodes in successive arrays around a cathode to provide a magnetron type tube structure, can provide a high current electron beam to a plurality of output electrodes in rapid succession and with great reliability. With reference to Figure 1 it is seen that the tube 20 is surrounded by a cylindrical magnet 21, here shown only in section, which provides a strong axially aligned magnetic eld therethrough. The tube contains a central axial cathode Z2 surrounded by several concentric arrays of electrodes. On a circular locus nearest to the cathode are the beam forming and holding electrodes 23 called spade electrodes. Beyond the spade electrodes on another circular locus are target electrodes 24 positioned peripherally on this circle so as to cover the inter-spade spaces and to collect beam current ilowing into such spaces between spades. Intermediately positioned between one edge of each Spade and the near edge of each target near that spade are switching grid electrodes Z5. These grids also are on a circular locus centered on cathode 22.

With the cathode heated to electron emission temperature and positive potential on the spade electrodes and target electrodes, the magnetic eld from magnet 21 is above that value required for magnetron type cut-off of current in the tube and no current will flow. However, if one of the spade electrodes about half its previous potential the voltage gradients within the tube are distorted and a beam will form. This electron beam grazes the low potential spade and strikes the adjacent target electrode. A small portion of the electron beam does flow to this spade. With a suitable series resistor interposed in the connection of each spade to its positive voltage supply, this small portion of the is reduced in potential to l beam current produces an IR drop which enables the spade to hold the beam stably in place once it grazes that spade.

Switching grid electrodes 2.5, upset this'Y ,stable beam holding condition when suitable negative. potential is applied to. the grid which is in the same inter-spade space as is the beam. As this grid goes negative, the beam fans out across the target electrode and some of rit strikes the next spade electrode. This fraction of beam current produces another IR locus drop which lowers the potential ofV this next spade. With a lowered potentialon the next spade the beam'switches over to, the next target. This switching action occurs at a veryrapid rate. The time required to switch from one target to the next being in the order of one tenth of a microsecond.

Ecient operation of these tubes largely depends upon the strong axially aligned magnetic eld-,- aetu ally in the order of 400 gauss in some cases-produced bythe cylin-Y drical magnet 2,1, and as shown schematically by the ux lines 26 in Figure 3'. Also illustrated in this figure is the external magnetic tield 27 which surnounds the magnet making it impractical to position close tothe tube electronic or other magnetic'equipmentfindicated at 28-which might be adversely affected by the external magnetic eld. As shown in Figure 4, it is not sufcient to follow the usual shielding procedure by placing a can type shield 29 of low reluctance over the tube. VThis shield does confine this external magnetic eld to. an area closely adjacent to the tubel thus to prevent damage to the adjacent equipment 28, but it also provides a low reluctance path for the important internal flux, a substantial portion of which can Vby-pass the tube through the can walls, as indicated by ow lines 30.

In particular accordance with the concepts of the present invention thel ditlicnlty produced by introduction of the can shield is overcome by providing the .tube with low reluctance shields in the 4form of metallic end caps 31 and 32. As more clearly shown in Figure 2, end cap 31 positioned at the top of the tube is in the form of an inverted cup, it being understood, of course, that this is by way of example only and that the cap may take other suitable shapes, as desired. End cap 32 at the base of the tube is illustrated as a truncated cup, but it too may take other suitable shapes as desired. Peripheral edge portions 33 and 34 respectively of the caps lie adjacent to lthe top and bottom edges 3S and 36 of the magnet and preferably are imbedded in the potting compound 37 attaching the magnet to the tube.v Other methods of attaching the caps may be used, however, imbedd-ing them in the potting compound is quite adequate and inexpensive.

While it is necessary to provide an aperture 38 through bottom end cap 32 to accommodate tube pins 39, this does not adversely affect the eciency of this cap as the pins themselves cooperate with the cap to complete the low reluctance shield in this area.

Provision of these novel cap shields provides means producing a lower total reluctance in the desired path through the tubes as contrasted to the somewhat higher reluctance in the path through adjacently positioned structure, such as the can shield. More simply, and looking at it in a different light it can be said that the caps provide a vmagnetic barrier arranged to separate the useful axially directed ilux 26 from the useless external flux 27.

From the above it will now be understood that the present invention provides means permitting magnetron tubes to be used in close adjacency to other magnetic;v

apparatus without adversely atfectingsuch apparatus, or

4 more importantly, the axially aligned ux through the tube itself.

It is also evident that the end cap shields provide means preventing incident light from photo-electrically affecting the operation of these tubes. Then, too, the top shield is particularly useful in protecting the tube tip-oli identified by the numeral 41. While these latter advantages are incidental to the main purpose of the end caps they are nonetheless advantageous and important.

With reference to Figure 6 it will be seen that'the end caps might comprise a coating y42 sprayed or otherwise applied over the exposed ends of the tube, and while not necessary, it might be advantageous to extend the coating to cover the ends of the magnet, as shown at 43. It is obvious, of course, that such a coating would also provide a light shield and protection for the tip-oit 41.

What is claimed is: Y

1. A magnetron beam switching tube including a plurality of electron beam holding positions disposed about the longitudinal axis of the tube, each position including an electron beam holding electrode, a beam switching electrode and a target electrode, a cylindrical magnet open at its endsand enclosing said tube and providing an axially aligned ilow of magnetic ux through said tube, and magnetic shield means positioned at each end of said magnet and substantially closing the ends thereof.

2. A magnetron beam switching tube including a plurality of electron beam holding positions disposed about the longitudinal axis of the tube, each position including an electron beam holding electrode, a beam switching electrode and a target electrode, a cylindrical magnet open at its ends and enclosing said tube and providing an axially aligned ow of magnetic ux through said tube, a low reluctance solid magnetic shield member closing one end of said magnet and thereby enclosing one end of said tube, and a low reluctance magnetic shield at the other end of said magnetic shield `at the other end of said magnet and effectively closing said other end of said magnet.

3. A magnetron beam Vswitching tube including a plurality of electron beam holding positions disposed about the longitudinal axis of the tube, each position including an electron beam holding electrode, a beam switching electrode and a target electrode, a cylindrical Vmagnet open at its ends and enclosing said `tube and providing an axially aligned flow of magnetic tiux through saidftube, said magnet being secured to said tube by a potting material, and low reluctance magnetic shield members closing the opening ends of said magnet and having edge portions thereof embedded in said potting materialt 4. The tube defined in claim 3 wherein said shield members comprise metallic coatings on said tube and in contact with the ends of said magnet.

References Cited in the tile of this patent Y UNITED STATES PATENTS 1 2,163,589 Dallenbach ..r .lune 27, 1939 2,250,647 Miller g Iuly29, k1941 2,400,770 Mouromtsei May 21, 1946 2,551,798 Hansell e ,v May 8, 1951 2,591,997 Backmark Apr. -8, 19,52 2,712,097 Auwarter June 28, 19.55 2,721,955 Sin-pih Fan et al Oct. Y25,1955 2,755,403 Hickey f July 17,` 1956 2,764,711 Knchinsky T Sept.r25, 1956 2,781,476 Smith lFeb. 12, 1.957 2,804,568 Kuchinsky V a., Aug. 27,1957 

